Ferrite as three-way catalyst for treatment of exhaust gas from vehicle engine

ABSTRACT

A kind of ferrite used as a three-way catalyst for treatment of exhaust gas from vehicle engines is revealed. Mono-nitrogen oxides (NO X ), carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas from vehicle engines can be removed effectively by the three-way catalyst made from ferrite containing at least one substitutional metal.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a catalyst for treatment of exhaust gas from vehicle engines, especially to a three-way catalyst made from ferrite and used for effective treatment of exhaust gas from vehicle engines so as to overcome the high cost problem of a conventional catalytic converter caused by noble metal catalyst and improve removal efficiency of mono-nitrogen oxides (NO_(X)).

Descriptions of Related Art

Mono-nitrogen oxides (NO_(X)), carbon monoxide (CO), and hydrocarbons (HC) emitted from vehicles cause serious air pollution. Thus various countries worldwide have made regulations related to pollution and set emission standards. In recent decades, both the modification on fuel component and the engine combustion design already reach the limit yet can't satisfy the environmental requirement. The only feasible technique available now is the catalytic converter added in the exhaust pipe of vehicles. A kind of catalyst in the catalytic converter results in the simultaneous removal of CO, HC, and NO_(X) in the exhaust gas from vehicles and the catalyst is called a three-way catalyst. The typical three-way catalyst in the gasoline engine uses noble metals such as palladium (Pd), platinum (Pt), rhodium (Rh) as active centers. Pd and Pt are used as used as an oxidation catalyst that oxidizes CO and HC with oxygen to form carbon dioxide (CO₂) and water (H₂O). As to Rh, it is a reduction catalyst that converts NO_(X) into N₂ by using CO and HC in the exhaust gas as reductants. Non-selective reducing agents (like CO and H₂) are used so that the catalytic method for removing NO_(X) is called Non-Selective Catalytic Reduction (NSCR). In diesel engines, a reductant such as urea solution or ammonia is added for better removal of NO_(X). Since the reductant only reacts with nitrogen oxides, the method is called Selective Catalytic Reduction (SCR). The SCR catalyst available now removes NO_(X) only over a narrow temperature range. For example, noble metal catalysts react at 150-250° C. while V₂O₅/TiO₂ catalysts work at the range of 250-450° C. The temperature of the exhaust gas from vehicles changes along with vehicle driving state. Once the temperature of the exhaust gas is over the operating temperature range of the catalysts, the catalytic effect is dramatically lowered. Thus the conventional catalysts are unable to provide optimal performance for removal of NO_(X) under different conditions from cold starting to high speed running. Moreover, the noble metal catalyst is quite expensive owing to the noble metals with low production and high cost. This has negative effect on management of air pollution. Thus there is a need to provide an effective catalyst with lower cost and higher NO_(X) removal efficiency.

Ferrite has a spinel structure with a face-centered cubic lattice of oxide ions, represented by a general formula MO.M′₂O₃, wherein M is a divalent cation and M′ represents a trivalent cation. Once M are all ferrous ions and M′ are all ferric ions, the ferrite is magnetite FeO.Fe₂O₃ (commonly abbreviated as Fe₃O₄). Besides naturally synthesized, ferrite can also be prepared by wet chemical synthesis in aqueous or alcohol solution or by powder metallurgy in which solid-state reactions are performed at high temperature. In order to produce ferrite with different properties, various methods are modified. Instead of ferrous ions, non-ferrous divalent metal cations can be filled into the position of M by the synthesis technique. Non-ferric trivalent metal ions can also be filled into the position of M′ instead for replacement of ferric ions. Ferrite contains non-iron ions in the form of substitutional solid solution. The properties of ferrite are modified once the kind and the ratio of the substitutional metals are changed. This is beneficial to modifications of the catalytic properties. Ferrite can be used as a material for electromagnetic wave absorption, electromagnetic wave emission or for magnetic memory (such as magnetic disk, magnetic strip, etc.).

SUMMARY OF THE INVENTION

Therefore it is a primary object of the present invention to provide a catalyst made from ferrite containing at least one substitutional metal and used for removal of mono-nitrogen oxides (NO_(X)), carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas from gasoline engines or diesel engines efficiently.

It is another object of the present invention to provide a three-way catalyst made from ferrite containing at least one substitutional metal and used for treatment of exhaust gas from vehicle engines. The substitutional metal can be copper (Cu), manganese (Mn), titanium (Ti), cobalt (Co), zinc (Zn), nickel (Ni), strontium (Sr), calcium (Ca), magnesium (Mg), chromium (Cr), aluminum (Al), neodymium (Nd), samarium (Sm), lanthanum (La), and cerium (Ce), or their combinations. The molar ratio of the substitutional metal to iron is ranging from 1:0.5-1:10.

In order to achieve the above objects, a three-way catalyst made from ferrite for treatment of exhaust gas from vehicle engines according to the present invention includes at least one substitutional metal selected from copper (Cu), manganese (Mn), titanium (Ti), cobalt (Co), zinc (Zn), nickel (Ni), strontium (Sr), calcium (Ca), magnesium (Mg), chromium (Cr), aluminum (Al), neodymium (Nd), samarium (Sm), lanthanum (La), and cerium (Ce), and their combinations during synthesis while the molar ratio of the substitutional metal to iron is ranging from 1:0.5-1:10.

There is a plurality of ways for synthesis of ferrite including hydrothermal synthesis, ferrite process, co-precipitation method, sol-gel method, solid reaction method, etc. The ferrite process and the co-precipitation method are used for preparing ferrite in the present invention.

Ferrite is added with a binder for being extruded into a honeycomb catalyst body or being coated on a honeycomb metal/ceramic support to form a honeycomb catalyst body. After being dried, the honeycomb catalyst body is sintered in insert gas (nitrogen, argon gas or helium gas) to form a honeycomb three-way catalyst. The binder is produced by clay, aluminum oxide, Kaolinite, silicon dioxide, calcium sulfate, paraffin wax, etc.

The honeycomb three-way catalyst is loaded into a metal case to form a three-way catalytic converter. Then the three-way catalytic converter is connected between a gasoline engine and an exhaust port. A catalyst bed of the three-way catalytic converter is divided into a front catalyst bed and a rear catalyst bed. Secondary air is introduced between the front catalyst bed and the rear catalyst bed and the amount of the secondary air is about 0-30% of the exhaust gas.

The honeycomb three-way catalyst is placed into a metal housing to form a three-way catalytic converter. Then the three-way catalytic converter is connected between a diesel engine and an exhaust port. A catalyst bed of the three-way catalytic converter is divided into a front catalyst bed and a rear catalyst bed. Urea solution or ammonia is added between the front catalyst bed and the rear catalyst bed for SCR reaction so as to remove NO_(X).

According to research results of the inventor, it is found that NO_(X) is reduced into harmless nitrogen gas by various kinds of ferrite including Cu-ferrite, Mn-ferrite, Co-ferrite etc under low oxygen concentration while carbon monoxide or hydrocarbon is used as reducing agent. The gasoline engine is a fuel-rich combustion system so that exhaust gas from the gasoline engine contains very low concentration of oxygen and large amount of carbon monoxide and hydrocarbons. The ferrite catalyst of the present invention can achieve the above reaction by using exhaust gas components for removal of mono-nitrogen oxides (NO_(X)).

Even at a very low oxygen concentration, the oxygen gas can react with carbon monoxide or hydrocarbons in the presence of ferrite such as Cu-ferrite, Mn-ferrite and Co-ferrite. The gasoline engine is a fuel-rich combustion system and exhaust gas from the gasoline engine contains very low concentration of oxygen and large amount of carbon monoxide and hydrocarbons. Thus oxygen gas is running out owing to mono-nitrogen oxides removed in the front catalyst bed. There is a need to introduce a little amount of secondary air between the front catalyst bed and the rear catalyst bed for oxidizing and removing residual carbon monoxide and hydrocarbons clearly. The reaction occurs at the temperature of the exhaust gas.

Based on research results of the inventor mentioned above, it is learned that ferrite has catalytic activity for the reduction of mono-nitrogen oxides and the oxidation of carbon monoxide and hydrocarbons at the same time. Thus ferrites of the present invention can be applied to a three-way catalyst for removal of exhaust gas from the gasoline engine.

The diesel engine is a fuel-lean combustion system so that exhaust gas from the diesel engine has excess oxygen. Urea solution, ammonia, hydrogen or other selective reductant is required to inject into the engine for SCR reaction and further removal of mono-nitrogen oxides. According to research results of the inventor, it is found that various kinds of ferrite including Cu-ferrite, Mn-ferrite etc. can accelerate SCR reaction effectively within a wide temperature range 50° C.-400° C. The ferrite of the present invention features on low temperature starting, wide temperature range for optimal catalytic effect and improved NO_(X) removal efficiency compared with conventional techniques. Moreover, the ferrite has catalytic activity for the oxidation of carbon monoxide and hydrocarbons as mentioned above. The ferrite of the present invention can be applied to a three-way catalyst for removal of exhaust gas from the diesel engine.

Without using noble metals such as palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), etc, the three-way catalyst made from ferrite has advantage of lower cost. Moreover, the SCR reaction is initiated at the temperature as low as 50° C. while dealing with exhaust gas from the diesel engine. The effective operating temperature range of the ferrite catalyst is ranging from 50° C. to 400° C., which covers the varying temperature range of the exhaust gas from the vehicle engine. Thus the removal efficiency of mono-nitrogen oxides (NO_(X)) is significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein:

FIG. 1 is a scanning electron microscope (SEM) image of Cu-ferrite catalyst of an embodiment according to the present invention;

FIG. 2 shows clearance effect of Cu-ferrite catalyst of an embodiment on carbon monoxide (CO) under different oxygen concentration according to the present invention;

FIG. 3 shows clearance effect of Cu-ferrite catalyst of an embodiment on hydrocarbons (HC) under different oxygen concentration according to the present invention;

FIG. 4 shows clearance effect of Cu-ferrite catalyst of an embodiment on mono-nitrogen oxides (NO_(X)) under different oxygen concentration according to the present invention;

FIG. 5 is a schematic drawing showing structure of a three-way catalytic converter for treatment of exhaust gas from gasoline engines of an embodiment according to the present invention;

FIG. 6 shows clearance effect of a three-way ferrite catalytic converter on mono-nitrogen oxides (NO_(X)), carbon monoxide (CO) and hydrocarbons (HC) of an embodiment according to the present invention;

FIG. 7 shows catalytic effect of Cu-ferrite and Mn-ferrite on Selective Catalytic Reduction (SCR) for removal of mono-nitrogen oxides (NO_(X)) according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A kind of ferrite of the present invention is applied to a three-way catalyst for treatment of exhaust gas from vehicle engines. The ferrite containing at least one substitutional metal is used to remove mono-nitrogen oxides (NO_(X)), carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas from gasoline engines or diesel engines.

The ferrite of the present invention includes at least one substitutional metal selected from copper (Cu), manganese (Mn), titanium (Ti), cobalt (Co), zinc (Zn), nickel (Ni), strontium (Sr), calcium (Ca), magnesium (Mg), chromium (Cr), aluminum (Al), neodymium (Nd), samarium (Sm), lanthanum (La), and cerium (Ce), and their combinations during synthesis while the molar ratio of the substitutional metal to iron is ranging from 1:0.5-1:10.

The ferrite containing the substitutional metal such as Cu-ferrite can be prepared by the following ferrite process. (a) Mix a solution containing ferrous ions (such as ferrous sulphate solution or ferrous chloride solution) and a solution containing copper ions (such as copper sulfate solution, copper nitrate solution, or copper chloride solution) to get a mixed solution. (b) Adjust pH of the mixed solution to 7-14 and heat the mixed solution to 50-100° C. for a period of time. (c) During heating, oxygen or air is introduced into the mixed solution with adjusted pH to react and get a solid crude product. (d) Then separate, dry, grind and sieve the solid crude product to get Cu-ferrite powder. The present method can also be used to prepare the ferrite containing other substitutional metal. The ferrite containing at least two substitutional metals can be prepared by mixing solutions containing different substitutional metals in the step (a).

The ferrite containing the substitutional metal such as Mn-ferrite can be prepared by the following co-precipitation method. (a) Mix a solution containing ferric ions (such as ferric sulfate solution, ferric nitrate solution or ferric chloride solution), a solution containing ferrous ions (such as ferrous sulphate solution or ferrous chloride solution) and a solution containing manganese (2+) (Mn²⁺) (such as manganese sulfate solution, manganese nitrate solution, or manganese chloride solution) to get a mixed solution. (b) Heat the mixed solution to 70-100° C. in water bath and introduce nitrogen gas for several minutes. (c) Add ammonia solution into the mixed solution and keep heating the solution for 1-3 hours to get a solid crude product. (d) Then isolate, dry, grind and sieve the solid crude product to get Mn-ferrite powder. The present method can also be used to prepare the ferrite containing other substitutional metal. The ferrite containing at least two substitutional metals can be prepared by mixing solutions containing different substitutional metals in the step (a).

The ferrite powder can be further added with a binder for being extruded into a honeycomb catalyst body or being coated on a honeycomb metal/ceramic carrier to form a honeycomb catalyst body. The binder is produced by clay, aluminum oxide, Kaolinite, silicon dioxide, calcium sulfate, paraffin wax, etc. After drying, the honeycomb catalyst body is sintered at the temperature ranging from 200 degrees Celsius (° C.) to 800° C. in inert gas (such as nitrogen gas, argon gas or helium gas) to get honeycomb three-way catalyst.

Next the honeycomb three-way catalyst is loaded into a metal case to form a three-way catalytic converter. The three-way catalytic converter is connected between a vehicle engine and an exhaust port. A catalyst bed of the three-way catalytic converter is divided into a front catalyst bed and a rear catalyst bed. Secondary air is introduced between the front catalyst bed and the rear catalyst bed while dealing with the exhaust gas from the gasoline engine and the amount of the secondary air is about 0-30% of the exhaust gas. For treating the exhaust gas from the diesel engine, urea solution, ammonia, hydrogen or other selective reductant is injected between the front catalyst bed and the rear catalyst bed.

Embodiment One: Synthesis of Cu-Ferrite (Cu:Fe=1:2.5)

A method for preparing Cu-ferrite is provided in this embodiment.

Weight CuSO₄ and FeSO₄ accurately according to the molar ratio required (Cu²⁺:Fe²⁺=1:2.5). Place them into the reactor and add 1 L deionized water with stirring to dissolve CuSO₄ and FeSO₄ completely. Add 6N sodium hydroxide (NaOH) into the solution and adjust the pH of the solution to 9.5. Then heat the solution to 85° C. After the temperature and the pH value becoming stable, introduce air into the solution at 3 L/min volumetric flow rate. Maintain the reaction condition until oxidation reduction potential (ORP) meter readings turn and increase rapidly. A solid crude product, Cu-ferrite, is obtained.

Embodiment Two: Synthesis of Mn-Ferrite (Mn:Fe=1:2.5)

A method for preparing Mn-ferrite is provided in this embodiment.

Weight MnSO₄, FeCl₂ and FeCl₃ accurately according to the molar ratio required (Mn²⁺:Fe²⁺:Fe³⁺=1:0.167:2.333). Place them into the reactor and add 1 L deionized water with stirring to dissolve CuSO₄, FeCl₂ and FeCl₃ completely. Heat the solution to 80° C. in a water bath. Then introduce nitrogen gas into the solution for 5 minutes and add ammonium hydroxide into the solution to make the metals precipitate completely. Keep stirring and heating the solution for 2 hours to get a solid crude product, Mn-ferrite.

Refer to FIG. 1, the solid crude product is separated, dried, ground and sieved to get copper ferrite powder. The copper ferrite powder is observed by scanning electron microscope at 30000× magnification and each particle of the catalyst is formed by a plurality of nanoparticles with different sizes.

Embodiment Three: Discussion of Efficiency of Ferrite for Removal of Carbon Monoxide at Low Oxygen Concentration

Refer to FIG. 2, Cu-ferrite catalyst can completely convert CO to CO2 for removal of CO at the temperature lower than 160° C. under the condition that the oxygen concentration is only 1%.

Embodiment Four: Discussion of Efficiency of Ferrite for Removal of Hydrocarbons at Low Oxygen Concentration

Refer to FIG. 3, Cu-ferrite catalyst can still remove hydrocarbons clearly at the temperature lower than 270 t under the condition that the oxygen concentration is only 1%.

Embodiment Five: Discussion of Efficiency of Ferrite for Removal of Nitric Oxide

Refer to FIG. 4, Cu-ferrite can convert nitric oxide into nitrogen by using hydrocarbons in gasoline at the temperature lower than 270° C. for removal of nitric oxide.

Embodiment Six: Discussion of Efficiency of Cu-Ferrite for Removal of Mono-Nitrogen Oxides (NO_(X)), Carbon Monoxide (CO) and Hydrocarbons (HC) Exhausted from Vehicle Engines

(1) Cu-Ferrite Powder is Produced into Honeycomb Catalyst.

A preferred way is to coat Cu-ferrite powder with a binder on ceramic/or metal honeycomb support to form a honeycomb catalyst body. Another way is to mix Cu-ferrite powder with a binder to get a paste. Then the paste is produced into a honeycomb catalyst body by extrusion molding. The binder is produced by clay, aluminum oxide, Kaolinite, silicon dioxide, calcium sulfate, paraffin wax, etc. After being dried, the honeycomb catalyst body is sintered at 400° C. in inert gas such as nitrogen gas to get the honeycomb catalyst.

(2) Removal Test for Air Pollutants

Place the ferrite catalyst into laboratory equipment and introduce simulated exhaust gases formed by nitrogen, air, CO, NO and 95-gasoline for performing tests. Refer to FIG. 5, a catalyst bed of the three-way catalytic converter is divided into a front catalyst bed and a rear catalyst bed. Secondary air is introduced between the front catalyst bed and the rear catalyst bed and the amount of the secondary air introduced is about 15% of the volume of the exhaust gas. First carbon monoxide and hydrocarbons in exhaust gas are oxidized in the front catalyst bed so that the oxygen concentration of the exhaust gas is reduced to a very low level. Then mono-nitrogen oxides are reduced into nitrogen at low oxygen concentration by the ferrite catalyst and using carbon monoxide and hydrocarbons as reducing agents. At last, secondary air is introduced into a front end of the rear catalyst bed to burn residual carbon monoxide and hydrocarbons completely. Refer to FIG. 6, mono-nitrogen oxides (NO_(X)), carbon monoxide (CO) and hydrocarbons (HC) are removed at the temperature lower than 300° C. Thus the ferrite catalyst of the present invention is really an effective three-way catalyst.

Embodiment Seven: Discussion of Catalytic Effect of Cu-Ferrite and Mn-Ferrite on Selective Catalytic Reduction (SCR) of Mono-Nitrogen Oxides (NO_(X)) in Exhaust Gas from Diesel Engines

Refer to FIG. 7, experiment results show that Cu-ferrite and Mn-ferrite are effective catalysts for Selective Catalytic Reduction (SCR) of mono-nitrogen oxides (NO_(X)) within a temperature range of 50° C.-400° C.

In summary, the ferrite of the present invention not only uses as an effective three-way catalyst but also overcomes high cost problem of the conventional noble metal catalyst and improves NO_(X) removal efficiency. Moreover, the kinds of the substitutional metal contained in the ferrite of the present invention can be changed and the number of kinds of the substitutional metal contained in the ferrite is not limited. The front catalyst bed and the rear catalyst bed can be arranged with three-way catalysts made from different kinds of ferrite respectively.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A method for treating exhaust gas from vehicle engines by ferrite, comprising applying the ferrite used as a three-way catalyst to the exhaust gas from vehicle engines for removal of mono-nitrogen oxides (NO_(X)), carbon monoxide (CO) and hydrocarbons (HC).
 2. The method as claimed in claim 1, wherein the ferrite comprises iron and at least one substitutional metal selected from the group consisting of copper (Cu), manganese (Mn), titanium (Ti), cobalt (Co), zinc (Zn), nickel (Ni), strontium (Sr), calcium (Ca), magnesium (Mg), chromium (Cr), aluminum (Al), neodymium (Nd), samarium (Sm), lanthanum (La), and cerium (Ce), and their combinations; and wherein a molar ratio of the substitutional metal to the iron is ranging from 1:0.5 to 1:10.
 3. The method as claimed in claim 1, wherein the carbon monoxide (CO) and the hydrocarbons (HC) in exhaust gas are used as reductants for removal of the mono-nitrogen oxides (NO_(X)) in exhaust gas from gasoline engines.
 4. The method as claimed in claim 1, wherein urea solution, ammonia, hydrogen or other selective reductant is added for removal of the mono-nitrogen oxides (NO_(X)) in exhaust gas from diesel engines.
 5. The method as claimed in claim 1, wherein the ferrite is produced into a honeycomb catalyst.
 6. The method as claimed in claim 5, wherein the honeycomb catalyst is used as a three-way catalytic converter for treating exhaust gas from gasoline engines.
 7. The method as claimed in claim 5, wherein the honeycomb catalyst is used as a three-way catalytic converter for treating exhaust gas from diesel engines. 